Embodiments of the present invention relate to methods of forming powder metal materials and powder metal parts. More specifically, certain embodiments of the present invention relate to methods of forming powder metal materials and/or powder metal parts by densifying one or more surface regions of the materials and/or parts after sintering and prior to densifying one or more core regions of the materials and/or parts. Other embodiments provide powder metal parts, such as gears and sprockets, having surface regions and core regions having essentially full density. Still other embodiments related to brazed, welded, plated and gas-tight powder metal parts and components that can be made in accordance with the various non-limiting methods disclosed herein.
The mechanical properties of powder metal (“P/M”) materials tend to increase with increasing density. That is, as the density of a P/M material approaches the full or theoretical density of the material, the properties of the P/M material will approach those of the wrought material. As used herein, the term “theoretical density” means the true density of a material (or a part made therefrom) when fully densified into a product with no pores. Thus, as used herein, phrases “theoretical density of a powder metal material” or “theoretical density of a powder metal part” means the true density of the powder metal material (or part) when the material or part is fully densified into a product without any pores.
Conventional structural P/M materials made from iron-base alloys using single-press and sinter P/M forming processes typically yield P/M materials having densities ranging from 6.8 grams per cubic centimeter (“g/cc”) to 7.2 g/cc, depending upon the alloy. However, these densities are generally only about 86 to 92 percent of the theoretical density of the material.
Processes that have been used to increase the density of P/M materials and parts include, for example: double press and double sinter processes (“DP/DS”); high fatigue alloy processes (“HFA”), which are described in detail in U.S. Pat. No. 5,613,180, which is hereby specifically incorporated by reference herein; and powder forging processes (“P/F”). Examples of typical densities that can be achieved for P/M materials or parts made from iron-base alloys using one of the aforementioned processes are given below in Table I:
TABLE ITYPICAL COREPERCENT THEORETICALPROCESSDENSITIES (g/cc)DENSITYDP/DS7.3-7.592.8-95.3%HFA7.45-7.6594.6-97.2%P/F 7.7-7.8597.8-99.9%
While PM materials and/or parts produced via conventional DP/DS and HFA processes generally have the densities indicated in Table 1 in both the surface and the core regions of the part, PM materials and/or parts produced via conventional P/F processes generally have a slightly lower density in the surface regions of the part than in the core regions of the part. That is, core regions of conventionally powder forged P/M parts typically range from 97.8 to 99.9 percent of the theoretical density (or “percent theoretical density”), whereas surfaces typically have densities ranging from 97.8 to 99.0 percent theoretical density. As used herein, the term “core” or “core regions” means regions of the material or part interior to the surface. As used herein, the term “surface” means of the region of the material or part extending from an exterior of the material or part inwardly to a depth of about 0.040 inches. Further the term exterior refers to any externally disposed region of the material or part regardless of geometry. Thus, for example, the exterior of a tubular part includes the exterior defined by the outer wall of the tube, the exterior defined by the inner wall of the tube, and the exterior portions defined by the ends of the tube. As used herein, the term “depth” refers to the distance measured inwardly from an exterior of the part or material.
More particularly, in conventional P/F processing, a P/M part (or blank) that has been heated to around 1800° F. is placed into a forging die held at about 600° F. such that at least a portion of the surface of the part contacts the die surface during forging. Since the die is cooler than the part, the portions of the surface region of the part that are in contact with the die cool and becomes less formable than the interior of the part. As a result, forging does not increase the density of these surface portions to the same degree as the core regions of the part. Accordingly, these surface portions of the part typically exhibit more porosity than the core regions. This porosity is undesirable in certain applications as it creates areas of stress concentration during fatigue, which can result in premature fatigue failure.
One process that can increase the surface density of P/M materials and parts is surface rolling. In this method, a master, which has the desired final shape, is forced under pressure against a P/M part (or blank) which has been previously processed to a density of about 6.8 to 7.5 g/cc. Since the part is both lower in density and softer than the master, on contact with the master, the part can be conformed to the geometry of the master. At the points of contact with the master, the surface of the part can be densified to essentially full density or full density. As used herein, the terms “essentially full density” and “essentially fully dense” mean at least 98 percent of the theoretical density of the material (or part). As used herein the terms “full density” and “fully dense” mean greater than 99 percent of the theoretical density of the material (or part).
However, while the portion of the surface of a cylindrical part that is subjected to surface rolling can generally be uniformly densified to essentially full density (or full density) to depth ranging from to 0.010 inches to 0.020 inches, when parts having irregular surface geometries are subjected to surface rolling, the depth to which essentially full density (or full density) can be achieved can vary significantly within the densified portion of the surface. As used herein, the term “uniformly densified” means that at least 90% of the portion of the surface of a material or part that is subjected to densification is densified to the specified density and to the specified depth.
For example, when portions of the surface in the tooth root and/or flank regions of a gear are subjected to surface rolling, while some portions of the surface subjected to rolling can be densified to essentially full density (or full density) to a depth ranging from 0.010 inches to 0.020 inches, other substantial portions of the surface subjected to rolling can remain essentially undensified or achieve essentially full density (or full density) to a depth of less than 0.001 inches. In other words, the rolled surface is not uniformly densified to essentially full density (or full density) to a depth ranging from 0.010 to 0.020 inches. Accordingly, it can be difficult to attain sufficient depth and uniformity of densification by surface rolling when the part to be densified has irregularly shaped surfaces. Therefore, although surface rolling can be used to uniformly densify the surface of cylindrical P/M materials and parts, this method is less effective on irregularly shaped surfaces such as gear teeth, sprockets, and cams.
Further, although surface rolling as described above can increase the surface density of pressed and sintered P/M materials and parts, typically, the density of core regions of the P/M materials and parts remains the same as it was before rolling—i.e., 6.8 to 7.5 g/cc. While it is possible to densify one or more core regions of the PM materials and parts using one of the aforementioned processes (for example DP/DS, HFA, or P/F) prior to surface rolling one or more surface regions of the part, because the P/M material is relatively “hard” after these processes, surface rolling is generally not as effective in increasing the density of the surface of the part after these processes as immediately after sintering.
Additionally, while surface densification of P/M materials or parts, for example by surface rolling or shot peening, prior to sintering can have the effect of increasing the density of the surface of the materials or parts prior to further processing (such as by DP/DS, etc.), because the mechanical properties of the P/M materials or parts in the green state (i.e., unsintered state) are relatively low, surface densification prior to sintering is not practical for many applications. More specifically, subjecting a green P/M part to a surface densification process can result in cracking, breakage, or roughening of the part. This is particular true for parts having irregular features or small cross-sections, for example teeth or splines, which can be easily damaged during handling in the green state.